The invention relates to a method for obtaining fuel-loaded exhaust gases in a diesel engine.
This invention more particularly relates to motor vehicles having diesel engines equipped with an automatic, robotized or continuously variable torque (CVT) gearbox.
Such a method for obtaining fuel-loaded exhaust gases can, in particular, be implemented during a particulate filter regeneration phase in a motor vehicle comprising a particulate filter and an oxidation catalyst on its exhaust line.
In order to comply with the new pollution control regulations, it is known how to use a particulate filter, installed in the exhaust line downstream of the engine combustion chambers. This particulate filter retains the polluting particulates which accumulate therein with the running of the diesel engine.
The accumulation of particulates in the particulate filter causes clogging of the particulate filter which considerably hinders the removal of the exhaust gases from the engine. This causes a high backpressure in the exhaust line which affects engine performance.
In order to remove the particulate accumulated in the particulate filter, it is known how to periodically burn the particulates present in the particulate filter during phases called regeneration phases. For this purpose, the temperature of the exhaust gases is increased in order to increase the temperature of the particulates and to initiate their combustion.
One known solution for increasing the exhaust gas temperature is to adopt a particular injection method during the particulate filter regeneration phases. A first injection is carried out in the combustion chamber in the conventional way, that is, during the compression stroke of the piston movement in the combustion chamber, and then one or more additional injections, called delayed injections, are performed. In the context of the present invention, delayed injection means a fuel injection into the combustion chamber during the expansion stroke when the engine crankshaft has passed its position corresponding to the top dead center of the piston by an angle of between 30 and 40°.
A further or alternative solution consists in making one or more injections called late injections. In the context of the present invention, late injection means a fuel injection into the combustion chamber during the expansion stroke when the crankshaft has passed its position corresponding to the top dead center by an angle of between 100 and 130°.
By means of these late or delayed injections, it is possible to load the exhaust gases of the engine with fuel. This is because, like the fuel injected during a delayed injection, the fuel injected during a late injection is not completely burned in the combustion chamber. Part of the fuel reaches a catalytic portion of the exhaust line, conventionally provided for oxidizing residual hydrocarbons and carbon monoxide. The fuel is oxidized therein, thereby increasing the temperature of the gases in the oxidation catalyst, and downstream thereof. The gases thus heated can then be used for regenerating the particulate filter.
The methods comprising one or more delayed and/or late injections thereby serve to load the exhaust gases of a diesel engine with fuel, for example, to regenerate a particulate filter. More generally, they serve to increase the fuel concentration in the exhaust gases leaving the combustion chamber.
However, since the injection takes place during the expansion stroke, part of the fuel injected is practically sprayed directly on the liner of the cylinder which, with the piston and the cylinder head, bounds the combustion chamber. This fuel may then be diluted in the oil coating said liner. This dilution raises at least two problems. On the one hand, the lubricating oil flows along the cylinder liner to the lower crankcase of the engine. In this way, the diluted fuel soils the lubricating oil in the lower crankcase, thereby reducing the lubricating properties of the oil, and possibly causing serious problems in the engine and significantly shortening its life. On the other hand, this fuel being diluted in the oil is not expelled with the exhaust gases, thereby reducing the fuel content thereof to be burned in the oxidation catalyst.